The present disclosure relates to a spectrometry method for measuring a dimension in a film stack employing extinction coefficient modulation, and an apparatus for implementing the same.
Scatterometry is an optical model-based metrology technique used in semiconductor device manufacturing to monitor quality and compliance of device fabrication and to control process through automated process control. Scatterometry includes many types of optical metrology techniques such as ellipsometry, reflectometry, and varying angle scatterometry. A predictive database of optical diffractive signatures is developed prior to the measurement. Real-time diffraction signatures are compared with various models based on the predictive database to provide in-line measurement of device dimensions such as thickness of various layers.
In general, scatterometry solves an inverse-diffraction problem by searching the predictive database for a best fitting model that generates a diffraction signature similar to the measured diffraction signature from a real-time measurement target structure. One of the fundamental shortcomings of scatterometry is that a given diffraction spectrum could have come from various different shapes. This degeneracy is called correlation. In addition, some shape parameter variations can result is a minimal change in the diffraction signature, thereby providing an inherently large range for an estimated dimension provided by parameter fittings. This phenomenon is called parameter insensitivity
Further, some applications of scatterometry such as measurement of thicknesses of various layers with varying strain doping or a semiconductor material overfill with multiple materials having similar diffraction signature do not provide a reliable process feedback method since such in-situ doping processes do not provide any mechanism for feed-forward. Adjacent layers with similar dispersion characters have parameters that are inherently difficult to measure because of lack of scatterometry sensitivity, and thus, any measurement results in a large uncertainty in calculated thicknesses. Such limitations permeate all types of scatterometry. Further, this problem is exacerbated in 3-dimensional applications in which non-planar structures are measured due to limited parameter ranges that limit sensitivity.